In vehicle exhaust systems, it is desirable to provide a pipe joint with high pull-apart strength and with a good fluid seal between the pipes. For both lap joints and butt joints, a band clamp has been provided in the prior art which admirably meets the requirements of pull-apart strength and fluid sealing. However, in some applications of a certain type of high tension band clamps often referred to as the "TORCA.TM. clamp", there has been a problem in some cases of inadequate force or loading applied by the clamp to pipe joints. In other cases there is a problem of breakage of a clamping bolt during installation or service of the clamp. Applicant has discovered the cause of these problems and has discovered a solution which is realized in accordance with the invention set forth herein.
In the prior art, high tension band clamps of the TORCA.TM. type are disclosed in Cassel U.S. Pat. No. 4,312,526 granted Jan. 26, 1982 and Cassel et al. U.S. Pat. No. 4,629,226 granted Dec. 16, 1986. U.S. Pat. No. 4,312,526 is especially suited for either lap joints or butt joints in vehicle exhaust systems in which the clamp band is made of relatively thin material. The band clamp of U.S. Pat. No. 4,629,226 is especially adapted for lap joints in vehicle exhaust systems wherein the clamp band is made of relatively thick material.
Both of the high tension band clamps mentioned above are characterized by structure and operation as follows. The clamp comprises a clamp band which is disposed around a member, such as a pipe which is to be joined with another pipe. The clamp band is, for the most part, circular or roundish in cross-section and is adapted to fit around the pipe. The ends of the roundish sector terminate in a channel-shaped sector which comprises a pair of sidewalls extending radially outwardly from the roundish sector. The clamp band is made of a single piece of sheet metal and each free end thereof is folded back on itself to form a double layer. Thus, the sidewalls are of double thickness and terminate at their outer ends in respective loops or bights. The bights serve as retaining members for restricting the movement of the sidewalls when the tightening means is tightened. The clamp band is tightened around the pipe by tightening means which comprises a reaction member or spline which is disposed within the channel-shaped sector and which is adapted to seat upon the pipe. For this purpose, the spline has an inner surface of arcuate configuration conforming to the pipe. The spline is provided on each side with a laterally facing concave surface. The tightening means further includes a bolt which extends through the sidewalls and the spline. The threaded end of the bolt extends through a spacer or bar which is disposed outside one sidewall and which has a convex surface which is opposite one concave surface of the spline. The bolt has a head with a convex inner surface which is disposed outside the other sidewall opposite the other concave surface on the other side of the spline. Alternatively, a bolt with a conventional head may be used and a separate bar with a convex surface is disposed under the bolt head opposite the other concave surface of the spline. A nut on the threaded end of the bolt is tightened to draw the sidewalls toward engagement with the spline and thus develop high tension in the clamp band.
I have discovered that tightening of the bolt to seat the sidewalls against the spline causes the free ends of the sidewalls to be pulled toward the pipe with slippage relative to the spline which becomes firmly seated on the pipe during the tightening operation. This slippage causes the sidewalls to engage the bolt at the periphery of the bolt holes in the sidewalls and to pull the bolt toward the pipe. As a result, the bolt becomes firmly seated against the periphery of the bolt hole in the spline. Additional tightening of the bolt causes additional downward slippage of the sidewalls relative to the spline. This causes the two sidewalls to apply a bending moment on the bolt on opposite sides of the spline which acts as a fulcrum for the bolt. The retaining members on the sidewalls are adapted to limit the amount of slippage of the free ends of the sidewalls relative to the spline so that high tension can be developed in the clamp band when the tightening of the bolt is completed. Because of variation of dimensions of the parts within the allowable tolerances of manufacture, there will be some joints in which the bolt is subjected to bending stresses before the retaining members arrest the free ends of the sidewalls. In a pipe joint where the bolt is bent by the downward force of the two sidewalls pulling it into engagement with the spline, the bolt will be placed in tension across the upper part of its cross-section and it will be placed in compression across the lower part of its cross-section. Since only a fraction of the bolt cross-section is in tension the clamping force achieved by the clamp band is significantly less than the full potential. This can result in a joint with reduced pull-apart strength or reduced sealing. Also, with only a fraction of the cross-section of the bolt in tension, the tensile strength of the bolt is significantly reduced and the tensile forces which must be developed to tighten the clamp will exceed the tensile strength of the bolt and cause it to break.
A general object of this invention is to overcome certain disadvantages of the prior art and to provide a high tension band clamp with a reduced incidence of bolt breakage upon tightening of the clamp.